Though the geocentric hypothesis prevailed in the Middle Ages, there is plenty of evidence to show that the smallness of the earth in relation to the heavenly bodies was understood.[681] William of Conches had thought that the sun was eight times as big as the earth.[682] In the _Image du monde_ this theme is elaborated:[683] we are told that it would take more than a hundred years for a rock to fall from the heavens; that the earth is like a tiny star in comparison with the immensity of the cosmos and is one hundred and sixty-six and three-twentieths times smaller than the sun.[684] John of Holywood quoted Alfraganus (Al-Farghānī) to the effect that the smallest fixed star is larger than the earth[685] but that the dimensions of such a star are as but a point in the firmament. He argued that the extreme smallness of the earth is proved by the fact that it is possible to see the middle of the firmament (_medietas firmamenti_) not only from the center of the earth but also from the earth’s surface.[686] His argument, which certainly proves nothing as it stands, is evidently a confused reflection of Ptolemy’s reasoning in the _Almagest_.[687] As to the actual size of our planet various figures were occasionally quoted. The _De imagine mundi_[688] gives Posidonius’ estimate of the circumference as 180,000 stades, or 12,052 miles (_duodecies mille millaria et quinquaginta duo_). The _Image du monde_,[689] however, gives 20,428 miles. Eratosthenes’ 252,000 stades appears in Lambert of St. Omer’s _Liber floridus_[690] and John of Holywood’s _De sphaera_.[691] In the latter work it is cited on the authority of Ambrose, Macrobius, “et Eristenis philosophorum,” along with a brief account of the great Alexandrian geographer’s method of measurement. _ZONES, THE ANTIPODES, AND “CLIMATA”_ The surface of the terrestrial sphere, surrounded as it is by the heavens, is naturally subjected directly to the influence of the heavenly bodies. We must now examine those general phenomena of the globe as a whole which were conceived to be consequences of the earth’s shape and position in relation to the remainder of the universe, postponing for a later chapter the study of the more local features of the _oikoumene_ (or habited quarter), which also result from the same circumstances. ZONES The most primitive observation reveals the fact that the heavenly bodies in their course through the sky revolve around two points and mark out certain circles. Very elaborate and often admirable discussions of the celestial poles, Arctic and Antarctic circles, equator, tropics, and ecliptic, are to be found in the numerous astrological and astronomical works of our period.[692] The study of these matters was already a highly developed science, but except in its geographical bearing it does not fall within our province.